1. Technical Field
Exemplary embodiments of the present disclosure generally relates to a pressing member, a fixing device including the pressing member, and an electrophotographic image forming apparatus, such as a copier, a printer, or a facsimile machine, including the fixing device.
2. Related Art
Conventionally, image forming apparatuses employing the electrophotographic method such as a copier, a printer, and a facsimile machine typically include a rotary photoreceptor drum, a charger that charges the photoreceptor drum, a laser scanning unit that emits a laser beam and exposes the photoreceptor drum after being charged by the charger to form an electrostatic latent image, a development device that develops the electrostatic latent image on the photoreceptor drum with toner to form a toner image on the photoreceptor drum, a transfer device to transfer the toner image from the photoreceptor drum onto a recording sheet, and a fixing device that heats and fixes the toner image on the recording sheet when the recording sheet with the toner image passes through the fixing device.
Generally, the employed fixing method to fix the toner image onto the recording sheet includes a fixing member (e.g., fixing roller, fixing belt) and a pressure roller. The recording sheet with the toner image passes in between the fixing member and the pressure roller. The pressure roller contacts and presses the recording sheet to the fixing member and the fixing member heats the toner of the toner image adhering to the recording sheet. The toner of the toner image on the recording sheet is softened and pressed to the recording sheet. Accordingly, the toner image is fixed on the recording sheet. In the above-described fixing method, the melted and fixed toner image on the recording sheet contacts the fixing member. Thus, a release layer formed of a material having good releasing properties (e.g., fluorine-based resin) with a layer thickness in a range from approximately 5 μm to approximately 30 μm is formed on the surface of the fixing member. A roller method is conventionally employed for the fixing member though due to a need to form an elastic layer having sufficient elasticity to obtain appropriate heating time with respect to color images and following capability, a belt method is recently employed for the fixing member (for example, refer to U.S. Pat. No. 3,578,797).
Further, a method (fixing sleeve type) of providing a pressing member and a heating body fixed to a metal or a resin holder inside a heat resistant sleeve, and fixing the toner image on the recording sheet by heating the recording sheet via the heat resistant sleeve is devised. FIG. 3A is a schematic view of a configuration of a fixing device including the heat resistant sleeve 303, the elastic layer 302, the release layer 301, a halogen heater constituting the heating body 304, a pressing member 306 fixed to a holder 305, and a pressure roller 309 that forms a nip with the release layer 301. The pressure roller 309 includes a sponge elastic layer 307, and a second release layer 308 having carbon. The configuration of 3A separates a heat source (i.e., heating body 304) and the pressing member 306. An example of the fixing device of 3A is disclosed in JP-2011-59247-A. FIG. 3B is a schematic view of a configuration of a fixing device including the heat resistant sleeve 303, the elastic layer 302, the release layer 301, a plate shaped heating body 310 and a pressing member 306 fixed to a holder 305, and the pressure roller 309 that forms a nip with the release layer 301. The plate shaped heating body 310 and the pressing member 306 fixed to the holder 305 is provided within the heat resistant sleeve 303. The configuration of 3B integrates the heating body 304 and the pressing member 306. Accordingly, the configuration of the fixing device of 3B has a short heat-up time to reach a predetermined fixing temperature. An example of the fixing device of 3B is disclosed in JP-S63-313182-A. It is to be noted that the heating body 304 and the pressing member 306 do not have to be inside the same heat resistant sleeve 303. The configuration of 3C is a schematic view of a configuration of a fixing device including the pressing member 306 provided inside heat resistant sleeve 303, and the heating body 304 provided inside the opposite pressure roller 309. An example of the fixing device of 3C is disclosed in JP-3298354-B1 (JP-H8-262903-A). Accordingly, the configuration of the fixing device of 3C has good recording sheet releasability.
However, recording sheets of various sizes are generally employed in the above-described fixing devices. For example, when a narrow width size recording sheet of 182 mm width B5 Size or an envelope of 105 mm width is passed through a fixing device that is capable of handling a maximum size recording sheet of 216 mm width Letter Size, heat at portions of a fixing member at which the narrow width size recording sheet does not pass through is not absorbed by the narrow width size recording sheet. Thus, uneven heat distribution is significantly generated in an axial direction of the fixing member and abnormal temperature rise at portions corresponding to end portions of the narrow width size recording sheet occurs. Accordingly, problems of degradation of the opposite pressure roller, wrinkling of the narrow width size recording sheet, unevenness of image gloss, and high temperature offset of toner are generated due to unevenness of heat of a surface of the fixing member.
To resolve the above-described problems, a method of suppressing abnormal temperature rise at portions corresponding to end portions of a recording sheet by locally reversing a state of insulation of the heating body and surrounding members is publicly known. An example of the publicly known method is disclosed in JP-H6-67556-A. However, the publicly known method disclosed in JP-H6-67556-A requires a method to detect size of the recording sheet and a driving system and is high-cost. In addition, the method disclosed in JP-H6-67556-A has a complex mechanism and uncertainty regarding mechanical reliability remains. Further, the method disclosed in JP-H6-67556-A can be only applied to the configuration shown in FIG. 3B. The method disclosed in JP-H6-67556-A cannot be applied to the configurations shown in FIG. 3A and FIG. 3C constituting a pressing pad method.